1. Field of the Invention
The present invention is concerned with an apparatus to obtain a tomographic image of a detected matter by introducing radioisotopes (RI) into the matter and then detecting a radiation from the radioisotopes by means of a scintillation detector or the like, and particularly relates to a radiation site detecting apparatus to detect a site where the radiation has entered the scintillation detector.
2. Related Background Art
A positron CT apparatus is known as an example of such apparatus to obtain a tomographic image of a detected matter, and has been popular as a high-level medical technique to obtain a tomographic image of a living body in which positron emission nuclei are introduced into the living body and a distribution thereof is measured by a scintillation detector.
Generally, the positron CT apparatus is equipped with a scintillation detector having a number of scintillators arranged in a ring layout, in which the body dosed with the positron emission nuclei is set. Near a positron-emitted area, a positron emitted from the positron emission nuclei is combined with an electron to disappear and two 511 keV radiation rays (.gamma.-rays) are simultaneously emitted in opposite directions to each other. Thus, the annihilation site near the positron emission nuclei can be detected by the coincidence counting of these two radiation rays through two mutually opposed scintillatots. A tomographic image is obtained through image reconstruction based on the thus-detected data.
Further, in order to detect the annihilation site near the positron emission nuclei, a position of each radiation-entering scintillator (hereinafter, referred to simply as "incident scintillator") among a plurality of scintillators needs to be determined. The centroid position calculating method has been mainly applied in the conventional radiation site detecting apparatus.
FIG. 13 is a drawing to show an example of conventional radiation position detecting apparatus employing the above-described centroid position calculating method. The scintillation detector is provided with a scintillator array 1 having many scintillators arranged in a two-dimensional array, and two-dimensional-crossed-wire-position-sensitive photomultipliers arranged as opposed thereto on the back side of the scintillator array 1. In the two-dimensional-cross-wire-position-detection photomultipliers, wires in the X coordinate direction and in the Y coordinate direction are connected with interposed resistors. Further, an adder (adding circuit) 2x adds output signals x.sub.1 and x.sub.2 appearing at the both ends of the resistors in the X coordinate direction, and a divider (dividing circuit) 3x divides the output signal x.sub.1 by the addition result (x.sub.1 +x.sub.2) to obtain an X-coordinate position x.sub.1 /(x.sub.1 +x.sub.2) of an incident scintillator. On the other hand, an adder (adding circuit) 2y adds output signals y.sub.1 and y.sub.2 appearing at the both ends of the resistors in the Y-coordinate direction, and a divider (dividing circuit) 3y divides the output signal y.sub.1 by the addition result (y.sub.1 +y.sub.2) to obtain a Y-coordinate position y.sub.1 /(y.sub.1 +y.sub.2) of the incident scintillator.
However, even if Compton scattering occurs, the conventional radiation site detecting apparatus employing the centroid position calculating method calculates a centroid position of scintillations based on scattered positions. Therefore, it was impossible to precisely detect a scintillator which a specific radiation has first entered. It is general that the size of each scintillators in the scintillator array 1 shown in FIG. 13 is smaller in order to improve the spatial resolution of tomographic image. However, the size reduction of each scintillators would cause the following problem because of the Compton scattering in the radiation-entering scintillator. Scintillation occurs when part of energy is absorbed depending upon a scattered angle, but some Compton-scattered radiation could be scattered into another scintillator (different from the incident scintillator), resulting in scintillating in some scintillatots. Accordingly, if only the scintillation in the incident scintillator should be able to be detected using the two-dimensional-crossed-wire-position-sensitive photomultipliers, the spatial resolution of a tomographic image could have been improved. However, since scintillations also occur in some other scintillators, the centroid position calculating method inevitably causes a decrease in precision of position detection.